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New Member

Which election occur in DR/BDR selection.

As per Jeff Doyle's Routing TCP/IP book "when an OSPF router becomes active and discovers its neighbors, it checks for an active DR and BDR. If a DR and BDR exist, the router accepts them. If there is no BDR, an election is

held in which the router with the highest priority becomes the BDR. If more than one router has the same priority, the one with the numerically highest Router ID wins. If there is no active DR, the BDR is

promoted to DR and a new election is held for the BDR."

My questin is if my network (multi access) becoming active, which election will held first? whether DR election or BDR election.

Thanks in advance.

Regards,

Kannan.S.T.

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: Which election occur in DR/BDR selection.

If you have a 'clean' network, the routers which connects will start off with no DR and no BDR.

The routers will then form 2WAY-connections with each other. The election will then take place as follows:

- The router with highest priority and router-ID will become the BDR

- Because none of the routers thinks it is the DR, this newly elected BDR will be promoted to DR.

- Then the first step will be done again, but this time excluding the newly promoted DR. In effect, it will then be the router with the next-highest priority/router-ID combination. And then we have a new BDR.

The criteria the routers use to declare themselves DR or BDR is by election.. If you have two routers connected on a LAN, one will be DR and the other BDR. If you now introduce another router, it will accept these values. If you however merge this LAN with another LAN which already has a DR and BDR, there will be new elections to see who will be the new DR and BDR of the two merged LANs..

Now you have two BDR's, and the best one of these will be the new BDR (based on router priority/ID combination). And likewise for the DR's.

Did it help?

9 REPLIES
Purple

Re: Which election occur in DR/BDR selection.

Hi Kannan,

Both the DR and BDR are elected as part of the same DR election process (as per section 9.4 of RFC2328). There is no separate election process per DR or BDR. Note that DR/BDR elections are held whenever certain conditions are met (e.g. new router joins the segment etc). The result of an election may be that the previous DR or BDR are retained or it may be the case that a BDR is promoted to DR. Just because the result of the election process is no change to existing router roles does not mean that the election has not taken place.

Hope that helps,

Paresh

Silver

Re: Which election occur in DR/BDR selection.

They are elected at the same time, but the BDR is elected before the DR in this process. See section 9.4 of the RFC (http://www.faqs.org/rfcs/rfc2328.html).

Did it help?

New Member

Re: Which election occur in DR/BDR selection.

Hi John,

Thanks for your reply.

I unable to understand this,

"Only those routers on the list that have not

declared themselves to be Designated Router are eligible to become Backup Designated Router."

If in my network there is no DR and BDR, every router in my network will announce itself as DR and BDR or choose any one?

On what critiria basis the routers have declared themselves DR or BDR?

Regards,

Kannan.S.T.

Re: Which election occur in DR/BDR selection.

Hi Kannan,

There are 2 conditions to be fulfilled for this:

1 Priority greater than 0

2 Router transition to 2-way state.

If there is no DR and BDR, every router will declare itself as the DR or BDR.There will be a list prepared for the router which satisfy the above conditions and claim as DR or BDR.There will be a list of DR eligible routers and a list of BDR eligible routers.The router with the highest router ID becomes the BDR and if there is no active DR on the n/w, BDR is promoted as DR. After that as there is no DR perremtion on the n/w, any new router comes in and accepts the DR as the DR for the segment and forms tha adjacency with DR.

HTH,

-amit singh

Silver

Re: Which election occur in DR/BDR selection.

If you have a 'clean' network, the routers which connects will start off with no DR and no BDR.

The routers will then form 2WAY-connections with each other. The election will then take place as follows:

- The router with highest priority and router-ID will become the BDR

- Because none of the routers thinks it is the DR, this newly elected BDR will be promoted to DR.

- Then the first step will be done again, but this time excluding the newly promoted DR. In effect, it will then be the router with the next-highest priority/router-ID combination. And then we have a new BDR.

The criteria the routers use to declare themselves DR or BDR is by election.. If you have two routers connected on a LAN, one will be DR and the other BDR. If you now introduce another router, it will accept these values. If you however merge this LAN with another LAN which already has a DR and BDR, there will be new elections to see who will be the new DR and BDR of the two merged LANs..

Now you have two BDR's, and the best one of these will be the new BDR (based on router priority/ID combination). And likewise for the DR's.

Did it help?

New Member

Re: Which election occur in DR/BDR selection.

Hi John,

Thank you very much for your brief explaination. Now i am clear in this.

Regards,

Kannan.S.T.

New Member

Re: Which election occur in DR/BDR selection.

It will be always BDR first then DR.

Mano

New Member

Re: Which election occur in DR/BDR selection.

Hi Manohar,

As per RFC 2328,

"Only those routers on the list that have not

declared themselves to be Designated Router are eligible to become Backup Designated Router."

On what critiria basis the routers have declared themselves DR or BDR?

Regards,

Kannan.S.T.

New Member

Re: Which election occur in DR/BDR selection.

At firstly, all the routers attached to the same segment (broadcast or NBMA network) won't declare themselves DR or BDR (in Hello packets).

After electing the DR or BDR, some routers will declare themselves DR/BDR in their OSPF Hello packets.

Regards

Ying Lei

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